IT seemed unthinkable in the aftermath of 9/11, when 2,996 people were killed in the most devastating terrorist attack ever witnessed.

Now, however, six months before the rebuilt One World Trade Center tower reopens, firms are clamouring to move to possibly the most secure skyscraper on the planet.

Even before ribbons were cut two months ago when a spire was placed atop the 1,776ft, £2.5billion tower, firms were signing long leases.

They include publishing giant Conde Nast, which has paid in excess of £1billion for a 25-year lease, leading Manhattan legal and financial firms, and the US government’s powerful General Services Administration purchasing arm. China’s Vantone Industrial Co has paid a huge deposit for floor space which will combine business with a “cultural centre” aimed at tourists and US firms anxious to establish trade links with the Far East.

Developer Larry Silverstein expects “every square inch” of the world’s third tallest structure to be occupied by its official opening early next year.

Shatterproof: Offices have stunning views (PIC by Durst Organization)

It’s impossible to eliminate threat altogether

Head of security Doug Farber

He said: “We are somewhere between 65 and 75 per cent full. I don’t anticipate there will be one per cent left by the time the doors open.”

Head of security Doug Farber admits: “It’s impossible to eliminate threat altogether, but this building is all about managing threat, both in terms of construction and security.”

That security begins at ground level, with a base built to withstand a powerful blast and a concrete mix that is the strongest used for a New York City building.

The steel frame is just as tough. When the planes struck in 2001, floors below the impact buckled under the weight, causing a domino effect that led to total collapse.

The new One World Trade Center has beams and columns welded together to distribute weight.

If any two failed in a copycat attack, the rest would “pick up the slack”. Windows encasing the structure are shatterproof and seven traffic intersections outside will be manned by police guard stations.

Security and strength are paramount at the iconic £2.5billion skyscraper (PIC by Durst Organization)

“We are, in effect, building a fortress around One World Trade Center,” admitted Jeff Zupan, at the city’s Regional Planning Association.

Project manager Kenneth Lewis said: “People will be thinking about their personal safety. This building is iconic but not because it looks like a bunker. It’s iconic because it’s a beautiful building.”

As for safety, Stan Conroy, who oversaw every concrete floor being laid, said: “If we did get attacked again, I wouldn’t run from the building, I would run into it.”